Terahertz waves are electromagnetic waves with frequencies between 0.1 and 10 THz (wavelengths between 30 μm and 3000 μm). This wavelength range approximately corresponds to the infrared to far infrared region. Terahertz spectrometers using these terahertz waves have been developed.
The terahertz spectrometers are divided into a transmission type which irradiates terahertz waves on a sample and detects transmitted light, and a reflection type which irradiates terahertz waves on a sample and detects reflected light. The transmission-type spectrometers need samples to be in the form of a thin film of about 1 μm, since terahertz waves, which fall in the infrared to far infrared region, are strongly absorbed by most substances. Therefore, much attention has been paid to the reflection-type spectrometers which do not limit the thickness of samples.
As shown in FIG. 7, a conventional reflection-type spectrometer generates terahertz waves by pumping InAs 30 by an ultrashort pulse laser, irradiates the terahertz waves on a sample 35 by way of off-axis parabolic mirrors 31, 34, and makes reflected light incident on a photoconductive dipole antenna 37 by off-axis parabolic mirrors 34, 36 for photoelectric detection (See Kiyomi SAKAI et al, “Terahertz Time Domain Spectroscopy and Imaging”, The Review of Laser Engineering, Vol. 30, No. 7, July 2002, pp. 376-384). In this conventional reflection-type spectrometer, first light reflected by the sample 35 is photoelectrically detected and next, light reflected by a metallic mirror which is placed in the same position as the sample 35 is also photoelectrically detected for reference. Then complex amplitudes in the frequency domain obtained by computing the Fourier transform of the respective detected photoelectric waveforms are compared with each other so as to derive reflectivity and phase shift. The most important problem of this spectrometer is that an error occurs in phase shift unless measurement is conducted with the metallic mirror and the sample placed in exactly the same position. Besides, its samples are limited to solids. And liquid, amorphous living organisms or the like cannot be measured.
As mentioned above, the conventional reflection-type terahertz spectrometer requires the sample and the metallic mirror to be placed in the same position and has made large measurement errors. To decrease the measurement errors, these two objects need to be placed in the same position with high accuracy. This placement takes a lot of time and this spectrometer is poor in practicality. In addition, since its samples are limited to solids, this spectrometer is poor in general versatility.
The present invention has been made in view of the above problems of the conventional reflection-type terahertz spectrometer. It is an object of the present invention to provide a reflection-type terahertz spectrometer and spectrometric method in which a metallic mirror does not have to be placed in the same position as a sample and samples are not limited to solids.